在类定义上的构造函数 VS 上设置变量
最近我一直在想，在类定义上的构造函数 VS 上初始化具有默认值的变量是否有区别.
Lately I've been wondering if there's a difference between initializing the variables that have a default value on the Constructor VS on the class definition.
考虑到优化，哪个更好:
Which one is better, taking into consideration the optimization:
class TestClass
{
private $test_var = 'Default Value';
function __construct()
{
}
}
class TestClass2
{
private $test_var;
function __construct()
{
$this->test_var = 'Default Value';
}
}
推荐答案
在构造函数之外初始化属性的好处是，有人阅读你的代码会立即知道它的默认值.
The advantage of initializing properties outside of the constructor is that someone reading your code will immediatly know its a default value.
不便之处在于您不能以这种方式使用所有类型的数据——例如，不适用于对象实例化，或者我记得的 heredoc 语法.
Inconvenient is you cannot use every kind of data this way -- will not work with object instanciations, for instance, or with heredoc syntax, from what I recall.
我认为在性能方面没有太大区别 - 无论如何，在您的应用程序中可能有很多更重要的事情;-)
I don't think there is much of a difference when it comes to performance -- anyway, there are probably lots of things that matter a lot more, in your application ;-)
不过，纯粹是为了好玩，使用 Vulcan Logic Disassembler :
Still, purely for fun, using the Vulcan Logic Disassembler :
以第一个代码示例(temp-2.php):
With the first example of code (temp-2.php) :
<?php
class TestClass {
private $test_var = 'Default Value';
function __construct() {
}
}
$a = new TestClass();
你得到这些操作码:
$ php -d extension=vld.so -d vld.active=1 temp-2.php
Branch analysis from position: 0
Return found
filename: /home/squale/developpement/tests/temp/temp-2.php
function name: (null)
number of ops: 11
compiled vars: !0 = $a
line # op fetch ext return operands
-------------------------------------------------------------------------------
2 0 EXT_STMT
1 NOP
7 2 EXT_STMT
3 ZEND_FETCH_CLASS :1 'TestClass'
4 EXT_FCALL_BEGIN
5 NEW $2 :1
6 DO_FCALL_BY_NAME 0
7 EXT_FCALL_END
8 ASSIGN !0, $2
9 RETURN 1
10* ZEND_HANDLE_EXCEPTION
Class TestClass:
Function __construct:
Branch analysis from position: 0
Return found
filename: /home/squale/developpement/tests/temp/temp-2.php
function name: __construct
number of ops: 4
compiled vars: none
line # op fetch ext return operands
-------------------------------------------------------------------------------
4 0 EXT_NOP
5 1 EXT_STMT
2 RETURN null
3* ZEND_HANDLE_EXCEPTION
End of function __construct.
End of class TestClass.
虽然，使用第二个代码示例(temp-3.php):
While, with the second example of code (temp-3.php) :
<?php
class TestClass2 {
private $test_var;
function __construct() {
$this->test_var = 'Default Value';
}
}
$a = new TestClass2();
你得到这些操作码:
$ php -d extension=vld.so -d vld.active=1 temp-3.php
Branch analysis from position: 0
Return found
filename: /home/squale/developpement/tests/temp/temp-3.php
function name: (null)
number of ops: 11
compiled vars: !0 = $a
line # op fetch ext return operands
-------------------------------------------------------------------------------
2 0 EXT_STMT
1 NOP
8 2 EXT_STMT
3 ZEND_FETCH_CLASS :1 'TestClass2'
4 EXT_FCALL_BEGIN
5 NEW $2 :1
6 DO_FCALL_BY_NAME 0
7 EXT_FCALL_END
8 ASSIGN !0, $2
9 9 RETURN 1
10* ZEND_HANDLE_EXCEPTION
Class TestClass2:
Function __construct:
Branch analysis from position: 0
Return found
filename: /home/squale/developpement/tests/temp/temp-3.php
function name: __construct
number of ops: 7
compiled vars: none
line # op fetch ext return operands
-------------------------------------------------------------------------------
4 0 EXT_NOP
5 1 EXT_STMT
2 ZEND_ASSIGN_OBJ 'test_var'
3 ZEND_OP_DATA 'Default+Value'
6 4 EXT_STMT
5 RETURN null
6* ZEND_HANDLE_EXCEPTION
End of function __construct.
End of class TestClass2.
所以，我猜有一点不同......但不是那么重要^^
So, I'm guessing there is a bit of a difference... But not that important ^^
由你来解释操作码——但有趣的是在第一次转储中没有默认值"的痕迹......有趣，实际上^^
似乎 VLD 不能(或只是不能)转储所有内容:-(
Up to you to interpret the opcodes -- but the funny thing is there is no trace of 'Default Value' in the first dump... interesting, actually ^^
Seems VLD cannot (or just doesn't) dump everything :-(
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